Ssh – Executing script remotely with “curl | bash” feedback

bashcurlremoteremote-accessssh

I'm working on a platform that uses a lot such commands:
ssh login@server.com 'curl http://some_server/script.sh | bash'

This is really clean and handy to execute scripts remotely, however, I don't see anyway to get the output/exit code of the script. Anyone can figure out something to make sure the script has been properly executed (from the host launching the ssh point of view).

Best Answer

As @Zoredache points out, ssh relays the status of the remote command as its own exit status, so error detection works transparently over SSH. However, two important points require special consideration in your example.

First, curl tends to be very lenient, treating many abnormal conditions as success. For example, curl http://serverfault.com/some-non-existent-url-that-returns-404 actually has an exit status of 0. I find this behavior counterintuitive. To treat those conditions as errors, I like to use the -fsS flags:

  • The --fail flag suppresses the output when a failure occurs, so that bash won't get a chance to execute the web server's 404 error page as if it were code.
  • The --silent --show-error flags, together, provide a reasonable amount of error reporting. --silent suppresses all commentary from curl. --show-error re-enables error messages, which are sent to STDERR.

Second, you have a pipe, which means that a failure could occur in either the first or the second command. From the section about Pipelines in bash(1):

The return status of a pipeline is the exit status of the last command, unless the pipefail option is enabled (see The Set Builtin). If pipefail is enabled, the pipeline’s return status is the value of the last (rightmost) command to exit with a non-zero status, or zero if all commands exit successfully.

Side note: The bash documentation is relevant not because you pipe to bash, but because (I assume) it is your remote user's login shell, and would therefore be the program that interprets the remote command line and handles the execution of the pipeline. If the user has a different login shell, then refer to that shell's documentation.

As a concrete example,

( echo whoami ; false ) | bash
echo $?

yields the output

login
0

demonstrating that the bash at the end of the pipeline will mask the error status returned by false. It will return 0 as long as it successfully executes whoami.

In contrast,

set -o pipefail
( echo whoami ; false ) | bash
echo $?

yields

login
1

so that the failure in the first half of the pipeline is reported.

Putting it all together, then, the solution should be

ssh login@server.com 'set -s pipefail ; curl -fsS http://some_server/script.sh | bash'

That way, you will get a non-zero exit status if any of the following returns non-zero:

  • ssh
  • The remote login shell
  • curl
  • The bash at the end of the pipeline

Furthermore, if curl -fsS detects an abnormal HTTP status code, then it will:

  • suppress its STDOUT, so that nothing will get piped to bash to be executed
  • return a non-zero value which is properly propagated all the way
  • print a one-line diagnostic message to its STDERR, which is also propagated all the way
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